Static resistant electric initiator



Nov. 10, 1953 C. F. HORNE STATIC RESISTANT ELECTRIC INITIATOR FiledMarch 6, 1953 CHARLES FRED HORNE INVENTOR.

' AGENT.

Patented Nov. 10, 1953 STATIC. RESISTANT ELECTRIC INITIATOR Charles F.Home, Kingston, N. Y., assignor to Hercules Powder Company, Wilmington,Del., a corporation of Delaware Application March 6, 1953, Serial No.340,821

14 Claims.

This invention relates to electric initiators and more particularly toelectric initiators which are highly resistant to premature firing bystatic electricity.

The art has long recognized the dangers inherent in accidental dischargeof electric initiators by means of static electricity. Accidents, whichat the time of their occurrence have seemed without explanation, havebeen subsequently traced to the firing of an initiator by a staticdischarge. Since normally used ignition compositions necessarily arehighly heat sensitive, a discharge of relatively high voltage is quitecapable of igniting. the composition and firing the initiator. The arthas generally considered that such accidental firings result from adirect discharge from a lead wire to a metallic initiator shell in thelocus of the ignition composition. Obviously, caps can also be fired bydirect discharge through the bridge wire via the two lead wires. What isnot so well understood, however, is the fact that an initiator can alsobe fired by the passage of a static voltage through the bridge wire whenthe discharge is from shunted lead wires to shell at a point other thanthrough the explosives charge.

While the danger of premature firing due to static discharge is presentinv all types of electric initiators, the procedures employed inseismographic prospecting and the sensitive ignition compositionsemployed in seismic-type blasting caps have tended to make this type ofinitiator more susceptible to static than regular electric blasting capsand delay electric blasting caps. Until very recent years, commercialseismograph caps generally required a discharge in the order of 5,000volts to fire the cap by direct discharge through the ignitioncomposition and a discharge in the order of 12,000 volts to fir the capsby heating of the bridge wire when the charge was supplied by a 750micromicrofarad capacitor and the discharge was from shunted lead wiresto shell. It has been established that voltages of this magnitude may bedeveloped by a man under proper conditions and that much higher voltagesmay be generated in equipment employed by seismic prospectors such asdrill rigs and trucks. Actually, conditions favoring high voltage staticgeneration, such as low humidity, high winds, sandstorms, and the like,are common in the calities wher most seismic prospecting is done.Consequently, it will be seen that a. definite danger of accidentalfiring of seismic caps is present under such adverse conditions. As aresult of this danger, seismographic prospecting organizations andblasting cap manufacturers mercial blasting operations.

have been constantly attempting to raise the static resistance of allelectric initiators and especially that of the seismic-typ blasting cap.

A blasting cap structure has previously been proposed in which a leadwire is disposed in contact or almost in contact with the shell, at apoint removed from the ignition composition, in order to provide for adischarge of the static electricity from the lead wire tov the shell.However, this type of structure offers good protection from static orfiring of the cap only when the charge passes down the one lead wire tothe shell. Little or no benefit is obtained when the current passesthrough both lead wires. Even when both lead wires are so disposed,discharge will occur from. only one wire in many instances andprotection will be limited as far as heating of bridge is concerned.

A structure has also been proposed wherein one or both of the bared leadwires are connected to the shell by means of semiconductive material.outside the locus of the ignition composition. This construction affordsa considerable improvement in static resistance. However, it is verydifiicult with such a structure to maintain a proper balance ofconductivity that will allow a discharge from both wires to the shelland still have sufiicient resistance for protection against the lowvoltage currents. which attend many com- Additionally, it has been foundthat in some instances, the static resistance of this type of structurediminishes with storage. Furthermore, such caps have been .known to firewhen 10-40 volts from a battery are applied between lead wires andshell.

In another proposed structure, conductive material is disposed about thebared lead wires and extends to the shell. This conductive material actsas a true resistor in that the resistance is low, normally from 10-100ohms. The resistance of such a body of material is similar to that of aregular carbon resistor, being fairly constant, but subject to variationdue to temperature. The resistance does not change greatly due topassage of current until the currentv is suificient to cause heating.This type of cap gives good static protection in most instances.However, it has been found that in some instances discharges occur fromonly one wire which allow a firing of the cap by the heating of thebridge wire. Even more than in the case of the semiconductive material,this structure has a serious deficiency of insulation from shell to leadwires and can be fired in this manner with a very low voltage. In otherwords, this structure, while removing a considerable part of the hazardof static electricity, has introduced an equally undesirable hazard inthe form of undesirably low resistance between lead wires and shell.

In still another structure, it has been proposed to equip the bared leadwires with protrusions which extend toward the shell. This structure isusually employed with a matchhead ignition element which is insulated.However, a discharge usually takes place from only one wire and a firingof the cap by the heating of the bridge wire is thus permitted. Inaddition, a hotter spark is obtained when the discharge is directed fromlocalized points. Even though the protrusions are outside the locus ofthe ignition composition, such violent discharges are to be avoided whenthe same results can be obtained in a less violent manner.

While all of these preceding structures give an initiator a measure ofstatic resistance, it will be seen that in each case, the protectionfrom static is not complete and, in most instances, what protection isobtained is brought about by a structure which is characterized by lowvoltage breakdown. Furthermore, these structures are not characterizedby a sufiicient resistance to heating of the bridge wire by a staticdischarge through the bridge wire itself.

The object of the present invention, therefore, is an improved initiatorstructure which will result in greatly improved resistance to firing bystatic electricity through discharge from the lead wire to the shell. Anadditional object of the invention is an improved initiator structurewhich gives adequate protection against premature firing caused byheating of the bridge wire by the passage therethrough of a staticcurrent. A further object of the invention is an initiator structurewhich will give one or both of these desired results and which is alsocharacterized by high resistance to accidental firing due to straycurrents.

Generally described, the invention is a static resistant electricinitiator having, in combination, a metallic shell; an ignition assemblydisposed within the shell comprising a pair of lead wires joined attheir terminals by a bridge wire and an ignition composition disposedabout the bridge wire; and a body of semiconductive material disposedabout and in conductive relationship with bared portions of both leadwires at a point within the shell outside the locus of the ignitioncomposition, said body of semiconductive material approaching the innerwall of the shell but being separated therefrom by a distance less thanthat between either lead wire and the shell in the locus of the ignitioncomposition. By semiconductive body is meant a body which is aninsulator at low voltage, such as -100 volts, but an extremely lowresistance conductor at voltages comparable to that required for firingby static discharge. The resistance of such a body remains constant atpractically infinite value at the low voltages but suddenly becomes verylow when the higher voltage is applied. In a preferred embodiment thepresent invention will employ, as the semiconductive body, a conductivemetal powder such as aluminum, substantially uniformly dispersed in areadily moldable nonconductive substance, such as wax or sulfur.

According to a further preferred embodiment, the blasting initiator ofthe present invention will also be equipped with a bridge wire of high 4heat capacity in order to provide additional protection from a staticdischarge passing through the bridge.

The novel structure of this invention may be employed in any type ofelectric initiator to give substantially complete protection from thestatic discharges customarily encountered in blasting operations.However, in view of the fact that most difiiculty with staticelectricity is normally encountered with seismic-type caps, theinvention will be principally described and illustrated with referencesto seismic blasting caps.

Having generally described the invention, the novel structure thereofwill be more particularly illustrated with references to theaccompanying drawing in which Fig. 1 represents a sectional view of aseismic-type electric blasting cap and Fig. 2 represents a partsectional, part elevational view of a further embodiment of a similarcap. Like symbols refer to similar structural elements.

In Fig. l, a base charge II] of detonative explosive is pressed into thebase of a conductive metallic shell II. A priming charge I2 of primaryexplosive, such as a mixture of diazodinitrophenol and potassiumchlorate, is pressed upon the base charge II). A cavity ignition plug I3is positioned on the priming charge I2. The bared ends of insulated leadwires I4 pass through the ignition plug into the cavity I5 and theterminals thereof are joined by a bridge wire I6 of high heat capacity.Disposed in the cavity, I5 of the ignition plug I3 is a charge ofignition composition IT. The charge I! is butteredinto cavity I5 andcompletely surrounds the bared ends of the lead wires I4 and bridge wireI6. Positioned above the cavity ignition plug I3 and disposed aboutbared portions of the lead wires I4 is a cylindrical body I8 ofsemiconductive material. The body I8 is spaced from the inner wall ofthe shell II to form an annular gap I9 therebetween. A body of asphalticsealing material 2!] is disposed above the semiconductive body I8. A topseal of sulfur 2| is disposed above the body of asphaltic material 20.

In the structure described in the drawing, it

will be noted that the body of semiconductive,

material I8 is spaced from the inner wall of the shell I I by a distancewhich is substantially less than the distance between the lead wires andthe shell in the vicinity of the ignition composition. Thesemiconductive body acts as a nonconductor for the voltage employed tofire an electric initiator but is a good conductor for a high voltagestatic discharge. Therefore, while the semiconductive body I8 has noeffect on the normal operation of the cap, it does constitute a shuntfor high voltage discharges between the lead wires within the shell whenthe static voltage is applied to both lead wires. This effect is ofparticular benefit with a cap in which the bridge wire has beenaccidentally broken during manufacture. When the static voltage isapplied across either of the wires and the shell, the charge isconducted from the lead wire through the semiconductive mass I8 and arcsacross the gap between the semiconductive body I8 and the shell wall.Since the breakdown voltage of the semiconductive body I8 must be fairlylow in order to give the desirable protection against static discharge,the cap as a whole would have undesirably low voltage breakdowncharacteristics if the body I8 were to contact the shell wall as is thecase in several antistatic designs employed in prior art initiators.Because of the fact that the body is spaced from the shell wall, thevoltage breakdown value of the cap is greatly increased and offersadequate protection against the type of stray current which canreasonably be expected to be found in blasting operations. For bestresults it is preferred to space the semieonductive body from the shellwall by a distance of between 0.005 and 0.050 inch.

When the strength of the static charge results in a passage of thecurrent through the bridge wire, despite the shunt formed by thesemiconductive body I8, the use of a high heat capacity wire impedes thegeneration of sufficient heat in the bridge to ignite the ignitioncomposition II. The heat capacity of a bridge wire can be increased byincreasing the length or the diameter. The dimensions of an electricinitiator and manufacturing considerations place a limitation on thepermissible length of a bridge wire, and it is consequently preferred toincrease heat capacity by increasing the diameter. An increase indiameter will give the desired increase in protection against staticelectricity but for a given material the increase in diameter will alsoresult in a corresponding increase of the minimum firing current of thecap. While this fact has no bearing upon the reduction of staticsusceptibility of the initiator, it is desired to maintain the minimumfiring current of an electric initiator at a low level in theneighborhood of 0.5 to 1 amp. Therefore, if, in accordance with thepresent invention, the diameter of the wire is to be increased, it isdesirable to employ a metal or alloy in the bridge wire which ischaracterized by a high specific resistance and a high specific heat. Ithas been found that with most of the ordinary materials used by the art,the minimum firing current of the initiator is undesirably raised if thediameter of the bridge wire exceeds about 0.0015 inch, which is theusual diameter employed by the art. It has been found that greatlyincreased static resistance is obtained if a diameter of 0.0020 inch isemployed. It is preferred to employ a bridge wire diameter of about0.0025 inch and in order to avoid any substantial increase in minimumfiring current, it is preferred to employ a bridge wire made from anickel-cromium alloy, such as that manufactured under the trade namesTophet C, Ohmax, and Jellifi 1000.

In Fig. 2, an initiator is shown which is identical to that illustratedin Fig. 1, except for the provision of a body of dielectric material inthe form of a thin rubber sleeve 22 disposed about the semieonductivebody 18 and substantially filling the space between the wall of theshell Ii and the semieonductive body I8. Despite the dielectric natureof the sleeve 22, static still discharges through the thin rubber to theshell wall.

Although it is usually preferred to employ a conductive metal powder asthe conductive component of the semieonductive body, especially aluminumpowder, other particulate conductive materials, such as conductivecarbon may be employed. The nonconductive component which acts as abinder and carrier for the conductive particles is preferably wax. TheWax employed will desirably have a melting point of 120 F. or above inorder to prevent substantial cold flow of the wax subsequent tomanufacture. The melting point of the wax, or any other operablecarrier, will, of course, depend largely upon practical considerations.For instance, if the waterproofing material employed is an asphalticcomposition which is poured into the cap in molten form, a wax will beemployed which has a melting point substantially higher than thetemperature of the filling composition as it is poured into the shell.On the other hand, if a rubber or resin sealing plug is employed and isplaced in position in cold form, the only consideration then is to use awax which will not cold flow. If a molten resin is employed, the sameconsiderations then pertain as in the case of a molten asphalticcomposition. Preferred waxes include such natural waxes as candelilla,montan, and carnauba and synthetic waxes such as those manufacturedunder the trade names Ceramid, Acrawax, Acrawax C, Flexo Wax, and BarecoWax. Various mixtures of these waxes may also be employed. Of this groupAcrawax is preferred.

Instead of wax, however, rubber or rubber-like materials, resinousmaterials, sulfur and equivalent materials may be employed. Since it isdesirable that the body of semieonductive material be molded about thelead wires, readily moldable nonconductive materials are preferred.

It is preferred that the annulus between the semieonductive body and theshell be substantially unfilled except by air. If the shell is sealed bya molten material which hardens in situ, some of this material may flowinto the annulus. It has been found in practice, however, that thisseldom occurs, probably due to the fact that the air is not readilydisplaced. Improved static resistance is still obtained, however, evenwhen the annulus is filled with an insulatory material, since the staticwill still discharge through the thin layer. In Fig. 2, a thin rubbersleeve has been employed but other dielectric, insulatory materials maybe employed such as sulfur, asphalt, synthetic resins and the like.

The amount of a particular conductive filler employed in thesemieonductive body will depend on the conductivity of the particularmaterial and on the degree of static protection desired. The method offorming the semieonductive body about the lead wires will also dictatethe optimum quantity of particulate conductive filler employed. In thepreferred composition of wax and aluminum, it has been found that bestresults are obtainable when between and 70% of particulate aluminum isemployed. The upper limit of 70% is primarily dictated by the fact thatit is desired to employ a pourable mixture for molding. The use of morethan 70% of aluminum powder usually results in a mixture which isundesirably viscous. It has been found that adequate protection againststatic discharge from the lead wire to the shell through the ignitioncomposition can be obtained with about 60% of particulate aluminum.Lesser amounts of aluminum can be employed with very beneficial results.From all considerations a 35 mixture of Acre.- wax aluminum has beenfound to give excellent results and is preferred.

Example 1 Seismic-type caps were prepared similar to that shown in thedrawing except that no bridge wire was employed. The semieonductive bodywas formed from 67 powdered aluminum and 33% Acrawax and was moldeddirectly on top of the cavity ignition plugs. The diameter of thesemieonductive body was 0.20 inch and the inner diameter of the shellswas 0.259 inch. These caps were tested for static susceptibility incomparison with conventional seismic caps having platinum alloy bridgewires about 0.00135 inch in diameter,

and with commercially available seismic caps having a conductive rubberplug surrounding bared portions of the lead wires and contacting theshell walls. The tests were made with a charge supplied by a I50micromicrofarad capacitor discharged through he cap. Connections weremade with the externally shunted lead wires and the shell. The followingresults were obtained:

inch platinum alloy bridge the maximum nonfiring voltage was 5000 volts.For the commercial cap with the conductive rubber plug in contact withthe shell, the maximum nonfiring voltage was 6000 volts. For the capwith the 0.0025 inch nichrome bridge, the maximum nonfiring voltage was14,000 volts. All three caps had a firing current of about 0.4 to 0.5amp.

Conventional Cap Commercial Cap Invention We PP No. No. No.

Tested Shot Failed Tested Shot Failed Tested Shot Failed Example 2Example 4 The conventional cap, the commercial cap, and the cap of theinvention as employed in Example 1 (but equipped with Tophet C bridgewires 0.0025 inch in diameter) were subjected to a voltage breakdowntest to determine the resistance of each cap to discharge by straycurrents. In this test a variable A. C. voltage was applied between thelead wires and shell and gradually increased until breakdown occurred.

Caps from group I (platinum alloy bridge 0.00135 inch in diameter) andfrom group II (Nichrome bridge 0.0025 inch in diameter) were then testedin comparison by applying a static discharge supplied by a 750micromicrofarad capacitor connected from the shunted lead wires to onebared lead wire near the cap. The maximum nonfiring voltage for the capsin group I was 10,000 volts while the maximum nonfiring Conventional CapCommercial Cap Invention X i i pp 18 No. No. No.

Tested Shot Failed Tested Shot Failed Tested Shot Failed 1 Same 43 capstested at progressively higher voltage.

From Example 1, it is seen that a conventional cap can be consistentlyshot by a static discharge from the lead wire to the shell through theignition composition when that discharge is in the order of 5000 volts.On the other hand, the commercially available cap and the cap of theinvention are both protected against discharge from lead wire to shellof static of much higher voltages than those normally encountered in thefield. Example 2 illustrates, however, that the high degree of staticresistance of the initiator in accordance with the invention is obtainedwithout sacrificing to an undesirable extent the equally essential highvoltage breakdown characteristics.

Example 3 Two groups of seismic-type caps were prepared without thesemiconductive body. In group I, 2 platinum alloy bridge wire wasemployed which had a diameter of 0.00135 inch. Group II differed only inthe use of a nichrome bridge (Tophet C) 0.0025 inch in diameter. Thesecaps and the commercial cap used in Examples 1 and 2 were tested againstsusceptibility to firing by directing a gradually increasing staticdischarge directly through the bridge via the two lead wires. The chargewas supplied by a 750 micromicrofarad capacitor. For the cap with the0.00135 voltage for the caps in group II was 26,000 volts.

Example 3 illustrates that the conductive plug did not greatly increaseresistance to static discharge through the bridge even though the plugactually contacted the shell.

Example 4 demonstrates that when a static discharge of over 10,000 voltsis applied from shunted lead wires to one band lead wire near the shell,enough heat is generated in the bridge wire to shoot the conventionalcap. When the bridge wire of higher heat capacity is employed, however,over 26,000 volts are required to generate enough heat in the bridge tofire the cap. However, it has been found that the spaced semiconductivebody of the invention does assist in preventing the lower staticvoltages from passing through the bridge. 0n the other hand, a bridgewire of increased heat capacity will give the desired protection againstthis type of static discharge without a semiconductive plug of any type.Actually, insensitivity to discharge can be increased to almost anydesired degree by increasing the heat capacity of the bridge. Thepractical limiting consideration in this regard, however, is the effecton the firing characteristics of the cap.

Example 5 Gaps in accordance with the invention were Conventional CapCap of Invention X i i pp 16 No. No.

Tested Shot Failed Tested Shot Failed Example 6 The two groups of capsof Example 5 were further tested by discharging a static charge suppliedby a 750 micromicrofarad capacitor from the shunted lead wires to one.bared lead wire near the cap with the following results:

Conventional Cap Cap of Invention X i i pp No. No.

Tested Shot Falled Tested Shot Failed Examples 5 and 6 illustrate theexcellent results which are obtainable when the spaced semiconductivebody and high heat capacity bridge wire are combined. In particular,Example 6 shows that the two features complement each other inpreventing accidental firing due to heating of the bridge when staticdischarges of higher voltage are encountered.

Example 7 Seismic-type caps were made similar to that shown in thedrawing. The semiconductive body contained 37% of a 50-50 mixture ofFlexowax C and Acrawax C and 63% of particulate aluminum. Thesemiconductive body was 0.25 inch high and 0.18 inch in diameter. Thebridge wires employed were 0.00135 inch in diameter and were made fromplatinum alloy. The inner diameter of the cap shell was 0.259 inch.Static voltage supplied by a 750 micromicrofarad capacitor was appliedfrom the shunted leg wires to the shell.

Voltage i Failed Shot Example 8 The tests of Example '7 were repeatedwith caps differing only in that the semiconductive body had a diameterof 0.20 inch.

Testd Failed Shot Mmcncqlo Ewample 9 The tests of Example 7 Wererepeated with caps in which the semiconductive body was formed from a 3367 mixture of Acrawax aluminum, had a diameter of 0.22 inch and was 0.25inch in height. Four caps were tested at 26,000 volts and three caps at36,000 volts. All seven caps failed.

The foregoing examples clearly illustrate that the improved initiatorsof the present invention are characterized by enhanced resistance toaccidental firing by means of static discharge either between lead wireand shell or through heating of the [bridge as a result of the flowtherethrough of static voltage.

As will be apparent to those in the initiator art, the conventionalcomponents of the initiators may be replaced by equivalents. The shellsmay be of any conductive metal such as brass, copper, ferrous metals andvarious alloys. The lead wires may be made of any of the conventionalmaterials such as copper, and iron, and may be tinned if desired. Thelead wires may be insulated with any desired material such as cottonservings, rubber or various plastics.

The base charges may be formed from any secondary detonative explosivesuch as pentaerythritol tetranitrate, cyclonite, tetryl,trinitrotoluene, and the like, and may be cast or pelleted as well aspressed when the nature of the explosive permits. The priming charge maybe omitted if the base charge is capable of initiation by the action ofthe ignition composition. When a priming charge is employed as ispreferred, any primary explosive or mixture may be used such asdiazodinitrophenol, diazodinitrophenol-potassium chlorate, lead azide,lead styphnate, and mercury fulminate. Any of the known ignitioncompositions may also be employed such as finely divideddiazodinitrophenol-chlorate mixture, fulminates, leador tin-seleniummixtures, etc.

The ignition assemblies need not be the cavity ignition typeillustrated. Instead, matchheads or loose ignition charges may beemployed in accordance with conventional structures employed in theblasting initiator art. The shell may be sealed by more than one sealinglayer as in the drawing or else a single seal may be employed such as arubber sealing plug orv a cast or molded resin plug, and the shellsuitably crimped.

The semiconductive body need. not be placed immediately above theignition assembly as specifically illustrated but may be disposed aboutbared portions of the lead Wires anywhere outside the locus of theignition composition.

Since, as indicated, the initiators within the scope of the inventioncan be altered in many respects without changing their mode ofoperation, it is intended that the invention be limited only by thescope of the appended claims.

This application is a continuation-in-part of my copending applicationfor United States Letters Patent Serial No. 231,491, filed June 14,1951.

What I claim and desire to protect by Letters Patent is:

1. A static resistant electric initiator having in combination ametallic shell; an ignition assembly disposed within the shellcomprising a pair of lead wires connected at their terminals by a bridgewire and an ignition composition disposed about the bridge wire; and abody of semiconductive material disposed about and in conductiverelation with bared portions of both lead wires at a point within theshell outside the locus of the ignition composition, said body ofsemiconductive material approaching the inner wall of the shell butbeing separated therefrom by a distance substantially less than thatbetween either lead wire and the shell in the locus of the ignitioncomposition.

2. An electric initiator in accordance with claim 1 in which the body ofsemiconductive material comprises particulate conductive material inadmixture with a nonconductive binder.

3. An electric initiator in accordance with claim 1 in which the body ofsemiconductive material comprises particulate conductive metal powder inadmixture with a nonconductive binder.

4. An electric initiator in accordance with claim 1 in which the body ofsemiconductive material comprises particulate aluminum and wax.

5. An electric initiator in accordance with claim 1 in which the body ofsemiconductive material comprises from 60 to 70% of particulate aluminumand 40 to 30% of wax.

6. A static resistant electric initiator having in combination ametallic shell; an ignition assembly disposed within the shellcomprising a pair of lead wires connected at their terminals by a bridgewire and an ignition composition disposed about the bridge wire, saidbridge wire having a diameter of at least 0.0020 of an inch; and a bodyof semiconductive material disposed about and in conductive relationwith bared portions of both lead wires at a point within the shell andoutside the locus of the ignition composition, said body ofsemiconductive material approaching the inner wall of the shell butbeing separated therefrom by a distance substantiall less than thatbetween either lead wire and the shell in the locus of the ignitioncomposition.

7 An electric initiator in accordance with claim 6 in which the body ofsemiconductive material comprises particulate conductive material inadmixture with a nonconductive binder.

8. An electric initiator in accordance with claim 6 in which the body ofsemiconductive material comprises particulate conductive metal powder inadmixture with a nonconductive binder.

9. An electric initiator in accordance with claim 6 in which the body ofsemiconductive material comprises particulate aluminum and wax.

10. An electric initiator in accordance with claim 6 in which the bodyof semiconductive material comprises from 60 to 70% of particulatealuminum and 40 to 30% of wax.

11. A static resistant electric initiator having in combination ametallic shell; a base charge of secondary explosive; a priming chargeof primary explosive; a cavity ignition plug, said plug having baredends of insulated lead wires passing therethrough and terminating in thecavity, a bridge wire having a diameter of between 0.0020 and 0.0030 ofan inch joining the terminals of the lead wires; an ignition compositiondisposed within the cavity and about the bridge wire; a substantiallycylindrical, semiconductive mass containing from 60 to 70% ofparticulate alumi-- num and from to 30% of a wax melting above 120 F.disposed above the ignition plug and about bared portions of the leadwires, said mass having a diameter less than that of the plug and beingspaced not more than 0.050 of an inch from the inner wall of the shell;and a body of sealing material disposed above said semiconductive mass.

12. A static resistant electric initiator having in combination ametallic shell; an ignition assembly disposed within the shellcomprising a pair of lead wires connected at their terminals by a bridgewire and an ignition composition disposed about the bridge wire; a bodyof semiconductive material disposed about and in conductive relationwith bared portions of both lead wires at a point within the shelloutside the locus of the ignition composition, said body ofsemiconductive material approaching the inner wall of the shell butbeing separated therefrom by a distance substantially less than thatbetween either lead wire and the shell in the locus of the ignitioncomposition; and a body of dielectric material disposed between thesemiconductive body and the shell wall.

13. A static resistant electric initiator having in combination ametallic shell; an ignition assembly disposed within the shellcomprising a pair of lead wires connected at their terminals by a bridgewire and an ignition composition disposed about the bridge wire, saidbridge wire having a diameter of at least 0.0020 of an inch; a body ofsemiconductive material disposed about and in conductive relation withbared portions of both lead wires at a point within the shell andoutside the locus of the ignition composition, said body ofsemiconductive material approachmg the inner wall of the shell but beingseparated therefrom by a distance substantially less than that betweeneither lead wire and the shell in the locus of the ignition composition;and a body of dielectric material disposed between the semiconductivebody and the shell wall.

14. A static resistant electric initiator having in combination ametallic shell; a base charge of secondary explosive; a priming chargeof primary explosive; a cavity ignition plug, said plug having baredends of insulated lead wires passing therethrough and terminating in thecavity, a

bridge wire having a diameter of between 0.0020

and 0.0030 of an inch joining the terminals of the lead wires; anignition composition disposed within the cavity and about the bridgewire; a substantially cylindrical, semiconductive mass containing fromto of particulate aluminum and from 40 to 30% of a wax melting above F.disposed above the ignition plug and about bared portions of the leadwires, said mass having a diameter less than that of the plug and beingspaced not more than 0.050 of an inch from the inner wall of the shell;a body of dielectric material disposed between the semiconductive bodyand the shell wall; and a body of sealing material disposed above saidsemiconductive mass.

CHARLES F. HORNE.

No references cited.

